The present invention relates to the oxybromination of hydroxyaromatic compounds in the para position using a metal catalyst, and more particularly to increasing the yield of the desired brominated product and recycling the reagents.
Monocyclic dihydroxyaromatic compounds such as hydroquinone and dihydroxybiphenyls such as 4,4′-dihydroxybiphenyl (“biphenol”) have numerous uses in the chemical industry. For example, both compounds can be used in polymer preparation, notably in the preparation of polycarbonates, polysulfones and polyimides, especially polyetherimides.
There are various methods for the preparation of hydroquinone and biphenol. As examples of such methods, each compound can be prepared from p-bromophenol, hydroquinone by hydrolysis and biphenol by reductive coupling in the presence of a noble metal catalyst, a base and a reducing agent.
Brominated hydroxyaromatic compounds, as exemplified by p-bromophenol, can be prepared by the method disclosed in the related co-pending commonly assigned U.S. patent application Ser. No. 10/342,475 filed Jan. 16, 2003. Briefly, the method comprises reacting the precursor hydroxyaromatic compound with a simple ionic bromide in an acidic medium, such as hydrobromic acid, and oxygen in the presence of a copper catalyst. Furthermore, as disclosed in the related co-pending commonly assigned U.S. patent application Ser. No. 10/650,566 filed Aug. 28, 2003, elemental bromine may be used as the bromine source in the bromination reaction. Also, related co-pending commonly assigned U.S. patent application Ser. No. 10/650,567 filed Aug. 28, 2003 discloses that a metal catalyst, such as one or more compounds or complexes of Group IV-VIII transition metals of the Periodic Table of Elements, may also be used to catalyze the bromination reaction.
While the above approach provides additional efficiencies, synthesis of p-bromophenol by reaction of brominating reagents with phenol is often complicated by the formation of undesired byproducts, most notably o-bromophenol. Thus, improved methods continue to be sought. Previous approaches have involved altering the conditions of the oxybromination reaction (e.g. process parameters, nature and concentration of catalyst), in order to reduce the level of the undesired o-bromophenol isomer. However, no set of reaction conditions to date has successfully yielded a 100% selective reaction to the desired p-bromophenol isomer. Another approach involves the separation (by distillation) of the o-bromophenol isomer followed by its palladium catalyzed reduction to bromide salt and phenol. While this is a high yield process, losses of organic substrate are observed, and use and losses of an expensive precious group metal catalyst are incurred.
Furthermore, in the catalytic preparation of brominated hydroxyaromatic compounds, it would be economical, and thus, highly desirable, if the reagents used in the process could be recycled to prepare additional products. In particular, it would reduce costs significantly in large-scale manufacturing operations if the metal catalyst used in the oxybromination reaction could be recovered and reused. The present invention meets these needs.